In general, for a light-emitting diode, the light output depends on the quantum efficiency of the active layer and the light extraction efficiency. The higher the quantum efficiency of the active layer, the higher the light output of the light-emitting diode. Generally, the quantum efficiency of the active layer is increased by improving the quality of the epitaxial structure and the structural design of the active layer. In addition, as the light extraction efficiency increases, the light output of the light-emitting diode is enhanced. In order to improve the light extraction efficiency, efforts are made to overcome the significant photon loss resulting from total reflection inside the light-emitting diode after emission from the active layer, the absorbing of the substrate, and the light shielding of the metal. Generally speaking, the problem may be overcome by using a flip-chip LED or by using a transparent substrate.
Besides, since the use of LEDs for illumination has become increasingly popular in recent years and most applications of LEDs as illumination are of high efficiency, the substrate needs to be made of a material with a good heat-dissipating property. The flip-chip LED not only may overcome the photon loss resulting from the light shielding of the metal, but also may achieve the goal of high efficiency and high power by using materials with good heat-dissipating property, such as Si or AlN, as submount substrate. However, as far as the present production technique is concerned, the processes of the flip-chip LED are complicated and the yield is low.
Furthermore, although metal has an excellent heat conduction property and conductivity, the metal substrate is not suitable for an epitaxial substrate. Hence, wafer bonding is generally used to replace the original epitaxial substrate with a substitute substrate. However, the processes are also complicated and the yield is low, especially the yield loss resulting from the step of wafer bonding.